Each year 8 to 10 million people worldwide develop tuberculosis. Globally the incidence of tuberculosis is growing at a rate of 1% a year primarily due to rapid increase in disease prevalence in Africa. In other regions successful control efforts have begun to stabilize disease incidence. Nevertheless, approximately 2,000,000,000 people, equal to one-third of the world's population, are estimated to be infected with Mycobacterium tuberculosis bacilli, the microbes that cause TB. (World Health Organization 2006 Tuberculosis Facts).
The need to find new treatments or vaccination strategies for tuberculosis is stressed by the increasing worldwide HIV infection rate such that, presently, 250,000 TB deaths are HIV associated. Tuberculosis itself is the second largest killer of mankind with more than 2 million deaths occurring worldwide annually (World Health Organization 2006 Tuberculosis facts). The attenuated Mycobacterium Bovis bacillus Calmette-Guerin (BCG) vaccine is the only tuberculosis vaccine currently licensed for human use. The BCG vaccine is effective against severe pediatric and extra-pulmonary forms of tuberculosis. However, protection against adult pulmonary tuberculosis in developing countries is poor, with adult protection varying between 0 to 80% (Fine P. E. M., Lancet 2000; 346:1339-1345). The variable efficacy of tuberculosis vaccination appears to be geographically centered. For example in the United Kingdom approximately 75% protection has been observed (Hart P. D. and Sutherland I., BMJ, 1977; 2, 293-295). In contrast, clinical studies in India and Malawi failed to show consistent protection against pulmonary tuberculosis (Fine, P E, et al., Scand J Infec Dis, 2001; 33:243-45; Ponnighaus J. M., Lancet, 1992; 339:636-639).
As the only effective vaccine for TB is the BCG vaccine, current research efforts are focused on improving BCG efficacy (Dietrich G., Vaccine, 2003; 21:667-670). For example, recombinant BCG vaccine over expressing fusion protein of the antigen Ag85B, the early secreted antigen (ESAT-6) and IFN-γ increased specific antibody titers and cellular immune responses relative to standard BCG vaccine, recombinant BCG vaccine expressing Ag85B alone, or recombinant BCG vaccine expressing a fusion protein of Ag85B and ESAT-6 (Xu Y., FEMS Immunology and Medical Microbiology, 2007; 51:480-487). ESAT-6, a protein produced by virulent Mycobacterium tuberculosis, is absent in standard BCG vaccine strains and is currently undergoing intense study as a potential vaccine subunit against tuberculosis. For example, DNA vaccines encoding ESAT-6 combined with immunization with BCG in mice subsequently challenged with tuberculosis H37Rv showed improved ESAT-6 specific interferon gamma (Fan X., Scandinavian Journal of Immunology Oct. 4, 2007; 66:523-528).
In addition to studies of new subunit vaccines, prime-boost strategies are currently under investigation as a method of improving BCG immunogenicity (Goonetilleke N. P., Journal of Immunology 2003; 171:1602-1609; Kaufmann S. H., Nature Reviews Immunology 2001; 1:20-30). Prime-boost strategies commonly employ DNA vaccines. For example, when a DNA vaccine expressing Ag85B was administered in a murine M. tuberculosis model followed by boosting with BCG vaccine, improved protective efficacy over BCG vaccine alone was observed (Feng C. G., Infectious Immunology 2001; 69:4174-4176). Similarly, DNA injection encoding the M. tuberculosis proteins Apa, HSP-65 and HSP-70 subsequently followed by conventional BCG vaccination also improved protection against tuberculosis challenge in mice (Ferraz, Infection and Immunity 2004; 72:6945-6950).
Traditional immunizations are generally administered via an intramuscular or subcutaneous route. However, tuberculosis is primarily a respiratory disease. Thus, protection against infection and subsequent eradication of disease may best be accomplished by direct administration to the respiratory mucosa (Kallenius, et al. Tuberculosis (Edinb), 2007; 87:257-66). Intranasal vaccination may have advantages over other routes of administration such as, intranasal vaccination is not influenced by a preformed systemic immunity whereas parenteral vaccination is less effective in individuals with preexisting antibodies (van Savage J. M., Journal of Infectious Disease 1990; 161:487-492).
Circumventing the existence of preexisting antibodies is important in geographical regions where an improved vaccine against tuberculosis is most needed. Prior Th2 background immunity resulting from prior exposure to helminthes and saprophytic mycobacteria has been suggested to decrease the ability of BCG vaccine in inducing immunoprotection (Rook, Vaccine, 2005; 23:2115-2120). Further, it is envisaged that intranasal vaccination might be effective in preventing M. tuberculosis infections in the host (Kauffman S H., Nature Reviews of Immunology 2006; 6:699-704). Animal studies of intranasal vaccination showed increased protective efficacy as compared to subcutaneous route of vaccination (Giri, P K. et al. FEMS Immunology and Medical Microbiology, 2005; 45:87-93; Chen, L. et al. Infection and Immunity, 2004; 72:238-246).
While studies of live or killed BCG vaccine, protein subunit vaccines, recombinant bacterial vector vaccines, plasma DNA vaccines or combinatorial immunization approaches in both human and animal systems have been subjected to preliminary study, little is known as to which method produces the most robust immune response and the greatest level of protection in the subject. Further, detail concerning immune response characteristics induced by each vaccine type is yet to be fully elucidated. The increased prevalence of tuberculosis infection and increased resistance, particularly in the developing world, creates a need for an improved tuberculosis vaccine and vaccination strategy.